Pathogen ecologies and human interventions: The natural and unnatural histories of zoonotic diseases

Three diapered goats in the boot of a car in Bamako, Mali (photo on Flickr by Romel Jacinto/37 °C).

This week, the Lancet publishes a series of three papers on diseases that are ‘zoonotic’, that is, infections shared by people and other animals. As William Keresh of EcoHealth Alliance (New York) and his colleagues explain in the first paper, Ecology of zoonoses: natural and unnatural histories, the sharing of pathogenic organisms by people and wild and domesticated animals is a natural biological phenomenon that, ‘with present anthropogenic trends’ and without similarly integrated disease prevention and control responses, presents a clear and present danger to humanity.

‘More than 60% of human infectious diseases are caused by pathogens shared with wild or domestic animals. Zoonotic disease organisms include those that are endemic in human populations or enzootic in animal populations with frequent cross-species transmission to people.

‘Some of these diseases have only emerged recently. Together, these organisms are responsible for a substantial burden of disease, with endemic and enzootic zoonoses causing about a billion cases of illness in people and millions of deaths every year. Emerging zoonoses are a growing threat to global health and have caused hundreds of billions of US dollars of economic damage in the past 20 years. . . .’

In an accompanying commentary, Anatomy of a pandemic, Peter Daszak, also of EcoHealth Alliance, gives an overview of why zoonoses matter (and should matter) to us.

‘For millennia, human beings have been plagued by pathogens originating in other animal species. Pathogens that are now endemic in human beings, such as measles and smallpox, evolved from wildlife microbes that exploited our successful development for their own global spread.

‘Zoonotic diseases have had a substantial effect on our social, cultural, and economic development. When these diseases first began to emerge is unknown, but causal factors include large-scale ecological and demographic changes, such as the domestication of livestock and the formation of dense human populations around 10 000 years ago.

‘As human societies have developed, pathogens from animal hosts have continued to spill over into our population: the Justinian Plague (541—542 AD), the Black Death (first introduced into Europe in 1347), yellow fever in South America in the 16th century, the global influenza pandemic in 1918, and modern pandemics such as HIV/AIDS, severe acute respiratory syndrome (SARS), and triple-reassortant A H1N1 influenza.

‘The Lancet Series on zoonoses reassesses our relationship with zoonoses, and the human societal developments that drive their emergence. . . .

‘For example, the domestication of livestock that led to the emergence of measles is paralleled by more recent intensification of global food production that contributed to the emergence of variant Creutzfeldt-Jakob disease and other zoonoses. Expansion of road networks, development of agricultural land, and intensification of wildlife trade have caused novel pathogens to emerge from wildlife (eg, Nipah virus, SARS, and HIV). Furthermore, the expansion of trade routes, which contributed to the spread of Black Death in the 14th century and the emergence of smallpox in the Americas in the 16th century, has continued in the era of globalisation, with the concomitant spread of SARS, West Nile virus, influenza A H5N1, and monkeypox. We have become a dense globally connected network of human beings vulnerable to the rapid spread of new zoonoses. . . .’

This same week, IRIN, the humanitarian and news analysis service, highlights the role of urbanization in increasing the risk of zoonoses as people gets closer to animals (Predicting the next zoonotic pandemic).

‘Cities are growing, with roads and industries penetrating previously uninhabited wildlife habitats; some 3.3 billion people live in urban areas (cities and their outskirts), according to the UN. By 2030, the world’s urban population is expected to exceed five billion, with 80 percent located in the developed world. In places where animal “hosts” to disease start to disappear (as their habitats shrink), pathogens are finding a new home in human hosts.

Some 800 million people worldwide are engaged in urban agriculture, according to the World Bank, which identifies peri-urban livestock as a fast-growing sector that produces 34 percent of the world’s meat and nearly 70 percent of its eggs.

The Nairobi-headquartered International Livestock Research Institute (ILRI) has pointed out how urban livestock and agriculture can breed disease in some of the world’s most crowded places. In a recent survey in Dagoretti, one of eight districts of Kenya’s capital, Nairobi, the institute found up to 11 percent of households were affected by cryptosporidiosis, a diarrhoeal disease caused by a pathogen found in cattle, raw milk, soil, vegetables and contaminated water.